Extracting sub-bands from signals in a frequency domain

ABSTRACT

Embodiments are disclosed for extracting sub-bands of interest from signals in a frequency domain for transmission via a distributed antenna system. In one aspect, a transformed downlink signal is generated by performing a frequency transform on a downlink signal. The transformed downlink signal represents the downlink signal in a frequency domain. At least one sub-band of the transformed downlink signal is identified as including data to be transmitted via the distributed antenna system. The sub-band is extracted from the transformed downlink signal for transmission via the distributed antenna system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims priority to U.S. Provisional Application Ser. No.61/812,820 filed Apr. 17, 2013 and titled “Dividing Signals intoSub-Bands via Frequency Transforms,” the contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to telecommunications systemsand more particularly (although not necessarily exclusively) toextracting sub-bands of interest from signals in a frequency domain.

BACKGROUND

A distributed antenna system (“DAS”) can provide a signal transportnetwork for communicating signals between one or more base stations andmobile communication devices or other terminal devices. The DAS mayinclude master units and remote antenna units. The master units may beconnected to the base stations. The master units receive downlinksignals from the base stations and distribute downlink signals in analogor digital format to multiple remote antenna units. The remote antennaunits transmit downlink signals to mobile communication devices or otherterminal devices within coverage areas serviced by the remote antennaunits. In the uplink direction, the remote antenna units receive uplinksignals from terminal devices in the serviced coverage areas. The remoteantenna units may combine uplink signals and transmit the combineduplink signals to master units. The master units may transmit uplinksignals to the serving base stations.

A signal transport network provided by a DAS may be implemented usinganalog systems or digital systems. A digital system can include one ormore devices for digitizing analog downlink signals received from a basestation. A digital representation of the analog waveform is used totransmit the downlink signal via the DAS.

A master unit may route an entire downlink signal to remote antennaunits of the DAS. Routing an entire downlink signal to remote antennaunits can involve unnecessarily routing frequency bands in which novoice data or other data is transmitted.

It is desirable to distinguish sub-bands of signals communicated via aDAS that include voice or other data from sub-bands in which no voicedata or other data is transmitted.

SUMMARY

Certain aspects and features of the present invention are directed todistributed antenna systems that can extract sub-bands of interest fromsignals in a frequency domain.

In one aspect, a method for extracting sub-bands of interest fromsignals in a frequency domain for transmission via a distributed antennasystem is provided. The method involves generating a transformeddownlink signal by performing a frequency transform on a downlinksignal. The transformed downlink signal represents the downlink signalin a frequency domain. The method also involves determining that atleast one sub-band of the transformed downlink signal includes data tobe transmitted via the distributed antenna system. The method involvesextracting the sub-band from the transformed downlink signal fortransmission via the distributed antenna system.

In another aspect, a unit for extracting sub-bands of interest fromsignals in a frequency domain for transmission via a distributed antennasystem is provided. The unit can include a first interface section, aprocessor, and a second interface section. The first interface sectioncan receive a downlink signal from a base station. The processor cangenerate a transformed downlink signal by performing a frequencytransform on a downlink signal. The transformed downlink signalrepresents the downlink signal in a frequency domain. The processor canalso determine that at least one sub-band of the transformed downlinksignal includes data to be transmitted via the distributed antennasystem. The processor can also extract the sub-band from the transformeddownlink signal. The second interface section can provide the extractedsub-band to at least one remote antenna unit of the distributed antennasystem.

In another aspect, a distributed antenna system is provided. Thedistributed antenna system includes a unit and at least one remoteantenna unit. The unit can receive a downlink signal and generate atransformed downlink signal by performing a frequency transform on thedownlink signal. The transformed downlink signal represents the downlinksignal in a frequency domain. The unit can also determine that at leastone sub-band of the transformed downlink signal includes data to betransmitted via the distributed antenna system. The unit can alsoextract the at least one sub-band from the transformed downlink signal.The unit can also provide the extracted sub-band to the remote antennaunit. The remote antenna unit can generate a wireless RF signal based onthe extracted sub-band and transmit the wireless RF signal to a terminaldevice.

These illustrative aspects and features are mentioned not to limit ordefine the disclosure, but to provide examples to aid understanding ofthe concepts disclosed in this application. Other aspects, advantages,and features of the present disclosure will become apparent after reviewof the entire application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an example of a distributed antennasystem in which sub-bands of interest can be extracted from signals in afrequency domain according to one aspect of the present disclosure.

FIG. 2 is a flow chart depicting an example of a process for extractingsub-bands of interest from a downlink signal in the frequency domainaccording to one aspect of the present disclosure.

FIG. 3 is a block diagram depicting an example of a unit of adistributed antenna system that can sub-divide downlink signals intosub-bands in the frequency domain according to one aspect of the presentdisclosure.

FIG. 4 is a block diagram depicting an alternative example of a unit ofa distributed antenna system that can sub-divide downlink signals intosub-bands in the frequency domain according to one aspect of the presentdisclosure.

FIG. 5 is a block diagram depicting an example of a remote antenna unitof a distributed antenna system that can sub-divide uplink signals intosub-bands in the frequency domain.

DETAILED DESCRIPTION

Systems and methods are disclosed for dividing signals communicated viaa distributed antenna system (“DAS”) into equally spaced sub-bands usingfrequency transforms of the signals. In some aspects, a master unit orother unit of a DAS can transform a digital downlink signal from thetime domain to the frequency domain. The unit of the DAS can identifymultiple, equally sized sub-bands from the digital downlink signal inthe frequency domain. Dividing signals communicated via the DAS intoequally spaced sub-bands can thus reduce the amount of bandwidth used toroute the signals to different units of the DAS. For example, data forfrequency domain representations of individual sub-bands of interest canbe processed and routed to remote antenna units in the DAS. Data forfrequency domain representations of other sub-bands can be discarded orotherwise omitted from routing. Processing and routing individualsub-bands of interest can obviate the need to route the entire downlinksignal via the DAS.

In some aspects, a unit of the DAS can include an input section, aprocessor, and an output section. The unit can receive downlink signalsvia the input section and output downlink signals via the output sectionto a remote antenna unit or other unit in the DAS. The processor of theunit can generate a transformed downlink signal by performing afrequency transform on the downlink signal or by configuring one or moresignal processing devices to perform the frequency transform on thedownlink signal. Performing a frequency transform on the downlink signalcan include transforming the downlink signal from a time domain to afrequency domain. Non-limiting examples of a frequency transform caninclude a fast Fourier transform (“FFT”), a discrete Fourier transform,and a discrete cosine transform. The processor can determine that atleast one sub-band of the transformed downlink signal includes voice orother data to be transmitted via the DAS. The processor can extract,identify, or otherwise selected the sub-bands from the transformeddownlink signal that include the voice or other data to be transmitted.The output section can route the sub-bands having the data to one ormore remote antenna units or other units of the DAS.

In a non-limiting example, a master unit or other unit of a DAS canreceive signals from base stations and convert the downlink signals intodigital downlink signals. The master unit can decompose or otherwisesub-divide the digital downlink signals into multiple, equally sizedsub-bands. The master unit can extract or otherwise select sub-bands ofinterest from the equally sized sub-bands. Sub-bands of interest caninclude sub-bands in which voice data or other data is included. Themaster unit can provide sub-bands of interest to one or more remoteantenna units. A remote antenna unit receiving the sub-bands of interestcan convert the sub-band into a composite signal for transmission tomobile communication devices in a coverage area serviced by the remoteantenna unit. In some aspects, a master unit can route differentsub-bands extracted from a transformed downlink signal to different setsof remote antenna units.

Dividing signals communicated via a DAS into sub-bands can allow forseparation of individual sub-bands of interest from composite signalscommunicated via the DAS. For example, in a time domain, a compositesignal can be filtered to extract individual channels of interest,remove frequencies other than the channels of interest, or both.Transforming signals into a frequency domain representation of thesignals can allow for removing the individual frequency components orotherwise manipulating the signals in a more computationally efficientmanner as compared to filtering signals in the time domain. Also, eachsub-band of a signal that has been divided can include digital signalssampled at the same sampling rate. Dividing signals that have beentransformed into the frequency domain can provide greatercomputationally efficiency than extracting channels of interest usingthe signals in the time domain.

In some aspects, the signal power for one or more sub-bands or groups ofsub-bands extracted from a transformed downlink signal can be modifiedto increase or decrease the level of the signal in that sub-band. Forexample, the signal level of each sub-band or a group of sub-bands canbe compared to a threshold value. The signal level of a sub-band can bescaled up or down based on whether the signal level is above or belowthe threshold.

Detailed descriptions of certain examples are discussed below. Theseillustrative examples are given to introduce the reader to the generalsubject matter discussed here and are not intended to limit the scope ofthe disclosed concepts. The following sections describe variousadditional aspects and examples with reference to the drawings in whichlike numerals indicate like elements, and directional descriptions areused to describe the illustrative examples but, like the illustrativeexamples, should not be used to limit the present disclosure.

FIG. 1 is a block diagram depicting an example of a DAS 100 in whichsub-bands of interest can be extracted from signals in a frequencydomain. The DAS 100 can communicate signals between one or more basestations 101 a, 101 b and mobile communication devices via master unitsand remote antenna units of the DAS 100. The DAS 100 can include a unit102 and multiple remote antenna units 112 a-c.

The unit can be a master unit or other suitable unit that cancommunicate with one or more base stations 101 a, 101 b. The unit 102can receive downlink signals from the base stations 101 a, 101 b andtransmit uplink signals to the base stations 101 a, 101 b. Any suitablecommunication link can be used for communication between the basestations 101 a, 101 b and a unit 102, such as (but not limited to) awired connection or a wireless connection. A wired connection caninclude, for example, a connection via a copper cable, an optical fiber,or another suitable communication medium. A wireless connection caninclude, for example, a wireless RF communication link. The unit 102 cancombine downlink signals received from base stations 101 a, 101 b. Theunit 102 can transmit the combined downlink signals to one or more ofthe remote antenna units 112 a-c.

The remote antenna units can provide signal coverage in respectivecoverage zones 114 a-c. Providing signal coverage in the coverage zones114 a-c can include transmitting downlink signals received from the unit102 to mobile communication devices or other terminal devices in thecoverage zones 114 a-c. Providing signal coverage in the coverage zones114 a-c can also include receiving uplink signals from the mobilecommunication devices or other terminal devices in the coverage zones114 a-c. The remote antenna units 112 a-c can transmit the uplinksignals to the unit 102. The unit 102 can combine uplink signalsreceived from remote antenna units for transmission to the base stations101 a, 101 b.

Although FIG. 1 depicts a direct connection between the unit 102 and theremote antenna units 112 a-c, other implementations are possible. Insome aspects, the unit 102 can be connected to the remote antenna units112 a-c via one or more extension units or other intermediate devices.

The unit 102 can include a processor 104, an interface section 106, asignal processing section 108, and an interface section 110. Theinterface section 106 can include one or more physical layer (“PHY”)devices for communicating with base stations 101 a, 101 b. For example,the interface section 106 can include an external repeater, an internalRF transceiver included on an interface card, or other suitable RFinterface device to communicate with the base stations 101 a, 101 b. Theinterface section 110 can include one or more PHY devices forcommunicating with remote antenna units or other units of a DAS 100. Thesignal processing section 108 can include one or more modules forconditioning, filtering, combining, or otherwise processing signalsreceived via an interface section 106 and communicated to other devicesin the DAS 100 via an interface section 110.

The processor 104 can include any processing device or group ofprocessing devices configured to execute one or more algorithms foridentifying sub-bands of interest. The processor 104 can configure thesignal processing section 108 to sub-divide signals into equally spacedsub-bands. The processor 104 can configure the signal processing section108 to extract or otherwise select sub-bands of interest from theequally spaced sub-bands. The processor 104 can include any devicesuitable for executing program instructions stored in a non-transitorycomputer-readable medium or other memory device to control operation ofthe unit 102. Examples of processor 104 include a microprocessor, anapplication-specific integrated circuit (“ASIC”), a field-programmablegate array (“FPGA”), or other suitable processor.

The unit 102 can extract or otherwise identify sub-bands of interestfrom a downlink signal in the frequency domain. FIG. 2 is a flow chartdepicting an example of a process for extracting sub-bands of interestfrom a downlink signal in the frequency domain.

In block 210, the unit 102 generates a transformed downlink signal byperforming a frequency transform on a downlink signal received from abase station. One or both of the processor 104 and the signal processingsection 108 can execute computationally efficient algorithms forgenerating the transformed downlink signal and for determining theequally spaced sub-bands into which downlink signals can be divided.

In a non-limiting example, an FFT can be applied to a downlink signal toconvert the downlink signal from the time domain to the frequencydomain. One or more FFT modules or modules in the signal processingsection 108 of the unit can generate an FFT for a frequency spectrumused by the DAS 100. Each bin of the FFT can correspond to a sub-band ofthe downlink signal. Each bin can include information about the downlinksignal within the bandwidth of the bin, such as a magnitude and a phasefor the sub-band in the bin. A length of the FFT being used candetermine the bandwidth for each sub-band. For example, the processor104 can obtain a 1,024-point FFT of a signal received by the unit. Thesampling rate of the digital signal can be divided into the 1,024 FFTbins. The processor 104 can identify sub-bands of interest within thefrequency spectrum from the FFT of the frequency spectrum. The unit 102can use the processor 104 to extract the identified sub-bands forprocessing and routing.

In block 220, the processor 104 determines that at least one sub-band ofthe transformed downlink signal includes data to be transmitted via theDAS 100. The processor 104 can identify that the transformed downlinksignal includes sub-bands of interest. For example, the processor 104can determine that one or more bins of an FFT or other frequencytransform include data having a magnitude exceeding a thresholdmagnitude.

In some aspects, the processor 104 can analyze the downlink signal inthe time domain to determine if a signal of interest is present. Theprocessor 104 can calculate or otherwise determine which FFT binsinclude the signal of interest.

In additional or alternative aspects, the processor 104 can identifysub-bands of interest by examining the downlink signal in both the timeand frequency domain. The processor 104 can detect signalcharacteristics consistent with mobile wireless signals. For example,the processor 104 can detect a particular modulation format and therebyidentify signals of interest.

In additional or alternative aspects, the processor 104 can beconfigured by a user to extract specific spectral segments. Theprocessor 104 can determine the FFT bins corresponding to the configuredspectral segments.

In additional or alternative aspects, the processor 104 can communicatewith the base station 101 a, 101 b to determine which channels are usedby the base stations 101 a, 101 b. The processor 104 can identify theFFT bins that include the signals of interest based on which channelsare used by the base stations 101 a, 101 b. For example, the basestations 101 a, 101 b may transmit control signals that are addressed tothe unit 102 and that identify the channels being used. In some aspects,the control signals can be transmitted via the same communication linksas downlink signals that are to be communicated to terminal devices. Inother aspects, the control signals can be transmitted via a dedicatedcommunication link that is different from communication links used totransmit downlink signals to the DAS 100. The unit 102 can include aninterface card or other device used to communicate with one or more ofthe base stations 101 a, 101 b via the dedicated communication link.

In block 230, the unit 102 extracts the at least one sub-band from thetransformed downlink signal. In some aspects, the processor 104 canextract or otherwise select the sub-bands of interest by selecting FFTbins corresponding to the sub-bands of interest. Each bin can representa given amount of frequency spectrum. The unit 102 can transmit binscorresponding to the sub-bands of interest to one or more other remoteantenna units 112 a-c. In a non-limiting example, a portion of thedownlink frequency band may lack any voice data or other data to betransmitted. The unit 102 can remove sub-bands from the downlink signalthat correspond to the portion of the frequency band without voice ordata. Removing the sub-bands can reduce the occupied bandwidth on thetransport media between the unit 102 and the remote antenna units 112a-c.

In some aspects, selecting the bins of interest can cause one or morediscontinuities in the frequency domain that may result in distortion inthe time domain. Distortion can be minimized by filtering the sub-bandsof interest from the frequency spectrum used by the DAS 100. Forexample, the unit 102 can select bins corresponding to the sub-bands ofinterest by applying a windowing function to an FFT of the frequencyspectrum to minimize distortion in the time domain.

Extracting sub-bands of interest can differ from filtering a signal inthe time domain. Filtering signals may involve decimating the signal toreduce the sampling rate of the original signal in order to reduce theamount of bandwidth required to transport a digital version of thesignal. A receiving device may interpolate or otherwise up-sample thereceived signal. In the time domain, each sub-band may be down-convertedvia a mixer and decimated in a low-pass filtering operation. To combinemultiple sub-bands at an up-converter may involve tracking the phases ofthe down-converter and the up-converter. Tracking the phases of thedown-converter and the up-converter can involve identifying the delaysbetween the down-conversion process for de-composing or otherwisedividing the sub-band and the up-conversion process for reconstructingthe signal at a receiving device. Subdividing signal in the frequencydomain can obviate the need to track the phases of down-conversion andup-conversion processes. A phase relationship from bin to bin can beidentified by the process of transforming the signal into the frequencydomain.

In additional or alternative aspects, a filter can be applied to asignal in the time domain to sub-divide the signal into the sub-bandsprior to converting the signal to the frequency domain. For example, theunit 102 can include a filter bank or a set of discrete filters in thesignal processing section 108 prior to an FFT module or other frequencytransform module. The filter bank or a set of discrete filters can beused to sub-divide a downlink signal (either in analog or digitalformat) into multiple downlink signal components in the time domain.Each of the signal components can correspond to one of the sub-bands.The downlink signal components outputted by the filter bank or other setof filters can be transformed into the frequency domain using the FFTmodule or other frequency transform module. The bins of the frequencydomain representation that correspond to sub-bands of interest can beselected by the unit 102 and communicated via the DAS 100.

In additional or alternative aspects, using frequency transforms todivide downlink signals into sub-bands can allow the unit 102 to modifya signal level in some portions of the downlink frequency band relativeto signal levels in other portions of the downlink frequency band. Forexample, the unit 102 can modify a signal level for a portion of thedownlink frequency band by multiplying sub-bands within the portion ofthe downlink frequency band by an appropriate scaling factor.Multiplying sub-bands within the portion of the downlink frequency bandby the appropriate scaling factor can allow for simplified equalizationof signal levels from different signal sources.

In additional or alternative aspects, dividing downlink signals intosub-bands can allow for flexible routing and combining of signals frommultiple base stations 101 a, 101 b to different groups of remoteantenna units. Dividing downlink signals into smaller sub-bands allowsfor flexible routing of signals by identifying channels of interest orsub-bands of interest and focusing signal processing to those channelsof interest or sub-bands of interest.

In one non-limiting example, the unit 102 can determine that one or moresub-bands of a digital downlink lack data to be transmitted via the DAS100. For instance, the processor 104 of the unit 102 can identify one ormore bins of the frequency domain representation of the downlink signal(e.g., an FFT, a discrete Fourier transform, a discrete cosinetransform, etc.) that have signal level values that are less than adesired threshold. A signal level can include, for example, a signalpower, a voltage, a magnitude, a variance, or any other signal parameterthat is suitable for determining whether a signal is present. The unit102 can discard data from the identified bins or otherwise modify thefrequency domain representation of the downlink signal to exclude thesub-bands associated with the identified bins. The unit 102 can reducethe sampling rate of the modified digital downlink signal based on thesub-bands without data being excluded from the digital downlink signal.

In another non-limiting example, a unit 102 may receive downlink signalsfrom three base stations that transmit downlink in a common frequencyband. A first base station and a second base station may transmitdownlink signals using the same frequencies with the frequency band. Theunit 102 can route downlink signals from the first and second basestations to non-intersecting subsets of the remote antenna units 112a-c. For example, the unit 102 can route downlink signals from the firstbase station to one or more remote antenna units 112 a in the coveragezone 114 a and can route downlink signals from the second base stationto one or more remote antenna units 112 b in the coverage zone 114 b.The third base station may transmit downlink signals using frequencieswith the frequency band that are different from the frequencies used bythe first and second base stations. For example, the third base stationmay operate in frequencies that do not overlap the frequencies used bythe first and second base stations. Downlink signals from the first basestation and the third base station can be combined and sent to a subsetof the remote antenna units 112 a-c for transmission. Downlink signalsfrom the second base station and the third base station can be combinedand sent to a different subset of the remote antenna units 112 a-c fortransmission. For example, the unit 102 can route downlink signals fromthe first and third base stations to one or more remote antenna units112 a in the coverage zone 114 a and can route downlink signals from thesecond and third base stations to one or more remote antenna units 112 bin the coverage zone 114 b.

Downlink signals from base stations 101 a, 101 b can each be sampled ata rate that allows the downlink signals from the base stations 101 a,101 b to be represented in a combined signal sum. For instance, in theexample provided above, a unit 102 may receive downlink signals fromthree base stations that transmit downlink in a common frequency band.Downlink signals from the first base station and the second base stationmay be converted to a digital signal with a sampling rate of Xsamples/second. Downlink signals from the third base station may beconverted to a digital signal with a sampling rate of Y samples/second.Routing downlink signals from the three base stations to the remoteantenna units 112 a-c may involve sampling the downlink signals receivedfrom the base stations at a sampling rate of at least X+Ysamples/second. Communication links between the unit 102 and the remoteantenna units 112 a-c may require a total bandwidth of 3×(X+Y) forrouting the downlink signals from the three base stations to the remoteantenna units 112 a-c.

The bandwidth requirements for the DAS 100 described in the exampleabove can be reduced by identifying sub-bands of interest for thedownlink signals received from the three base stations. For example, theunit 102 can combine a first set of downlink signals from the first basestation and the third base station and can route the combined downlinksignals to a first subset of the remote antenna units 112 a-c. The unit102 can convert downlink signals from the first and third base stationsinto digital downlink signals using a sampling rate of 2×(X+Y) and cantransmit the digital downlink signals to the first subset of remoteantenna units 112 a over one or more communication links having abandwidth of 2×(X+Y). The unit 102 can also combine a second set ofdownlink signals from the second base station and the third base stationand route the combined downlink signals to a second subset of the remoteantenna units 112 a-c. The unit 102 can also convert the downlinksignals received from the second and third base stations into digitaldownlink signals using a sampling rate of 2×(X+Y) and can transmit thedigital downlink signals to the second subset of remote antenna units112 a over one or more communication links having a bandwidth of2×(X+Y). Sub-dividing downlink signals into sub-bands can thus providemore efficient use of resources in the DAS 100.

FIG. 3 is a block diagram depicting an example of a unit 102 that cansub-divide downlink signals into sub-bands in the frequency domain. Theunit 102 can include the processor 104, which is configured tocommunicate with signal processing devices in the signal processingsection 108. In a downlink direction, the unit 102 can include RFcircuitry 302 a, 302 b, analog-to-digital converters 304 a, 304 b, FFTmodules 306 a, 306 b, a combiner 310, framers 312 a, 312 b, and PHYdevices 314 a, 314 b. In some aspects, one or more of the FFT modules306 a, 306 b, the combiner 310, and the framers 312 a, 312 b can beimplemented as software modules executed by the processor 104. Inadditional or alternative aspects, one or more of the FFT modules 306 a,306 b, the combiner 310, and the framers 312 a, 312 b can be implementedusing dedicated signal processing circuitry, such as an FPGA. AlthoughFIG. 3 depicts two downlink paths for illustrative purposes, any numberof downlink paths can be implemented in the unit 102.

As depicted in FIG. 3, the interface section 106 can include the RFcircuitry 302 a, 302 b that is configured for receiving downlink signalsfrom the base stations 101 a, 101 b. Non-limiting examples such RFcircuitry include a wireless RF transceiver, an interface card forreceiving RF signals over coaxial cable or another wired connection,etc.

The signal processing section 108 can include the analog-to-digitalconverters 304 a, 304 b, the FFT modules 306 a, 306 b, and the combiner310. The analog-to-digital converters 304 a, 304 b of the unit 102 canconvert analog downlink signals received by the unit 102 to digitaldownlink signals. The FFT modules 306 a, 306 b can transform the digitaldownlink signals into the frequency domain. The processor 104 of theunit 102 can extract or otherwise select sub-bands of interest from thedownlink signals in the frequency domain. The processor 104 can becommunicatively coupled to some or all of the signal processing devicesof the unit 102 via any suitable structure for transporting electricalsignals between devices or components within the unit 104. For example,the processor 104 can communicate with the signal processing devices ofthe unit 102 via, for example, a printed circuit board (not depicted) orother conductive components that can be used to communicate electricalsignals within the unit 102. In some aspects, the processor 104 canprovide a control signal to the combiner 310 to ignore data in thefrequency domain received from one or more of the FFT modules 306 a, 306b other than the data in the frequency domain corresponding to thesub-bands of interest from the FFT module. In additional or alternativeaspects, the processor 104 can configure one or more of the FFT modules306 a, 306 b to only send bins corresponding to the sub-bands ofinterest to the combiner 310. In additional or alternative aspects, theprocessor 104 can configure one or more of the combiner 310 or one ormore of the framers 312 a, 312 b to only send bins corresponding to thesub-bands of interest to other devices or modules in the downlink path.The combiner 310 can combine the extracted sub-bands of interest fromthe downlink paths of the unit 102 into serialized downlink data fortransmission to remote antenna units. Although FIG. 3 depicts FFTmodules 306 a, 306 b for performing frequency transforms, any suitablefrequency transform device or module can be used in the downlinkdirection.

The interface section 110 can include the framers 312 a, 312 b and thePHY devices 314 a, 314 b. The framers 312 a, 312 b can packetizeserialized downlink data received from the combiner 310 for transmissionto one or more of the remote antenna units 112 a-c as a packetized datastream. The PHY devices 314 a, 314 b can transmit the packetizeddownlink data streams to the remote antenna units 112 a-c.

In an uplink direction, the unit 102 can include the PHY devices 314 a,314 b, deframers 316 a, 316 b, bin aligners 318 a-d, gain adjustmentmodules 320 a-d, bin summers 322 a, 322 b, gain adjustment modules 324a, 324 b, inverse FFT modules 326 a, 326 b, digital-to-analog converters328 a, 328 b, and RF circuitry 330 a, 330 b. In some aspects, one ormore of the deframers 316 a, 316 b, the bin aligners 318 a-d, the gainadjustment modules 320 a-d, the bin summers 322 a, 322 b, the gainadjustment modules 324 a, 324 b, and the inverse FFT modules 326 a, 326b can be implemented as software modules executed by the processor 104.In additional or alternative aspects, one or more of the deframers 316a, 316 b, the bin aligners 318 a-d, the gain adjustment modules 320 a-d,the bin summers 322 a, 322 b, the gain adjustment modules 324 a, 324 b,and the inverse FFT modules 326 a, 326 b can be implemented usingdedicated signal processing circuitry, such as an FPGA. Although FIG. 3depicts two uplink paths for illustrative purposes, any number of uplinkpaths can be implemented in the unit 102.

In an uplink direction, the interface section 110 for communicating withthe remote antenna units 112 a-c can include the PHY devices 314 a, 314b and the deframers 316 a, 316 b. The PHY devices 314 a, 314 b canreceive packetized uplink data streams from one or more of the remoteantenna units 112 a-c. The de-framers 316 a, 316 b can extract uplinkdata from packetized uplink data streams received from remote antennaunits 112 a-c.

The signal processing section 108 can also include the bin aligners 318a-d, the gain adjustment modules 320 a-d, the bin summers 322 a, 322 b,the gain adjustment modules 324 a, 324 b, the inverse FFT modules 326 a,326 b, and the digital-to-analog converters 328 a, 328 b. Each of thebin aligners 318 a-d can align frequency bins from FFT's or otherfrequency transforms of uplink signals. For example, FFT data for thesame uplink signals that are received by the unit 102 from differentremote antenna units 112 a-c may be shifted in time with respect to oneanother. Each of the bin aligners 318 a-d can be used to account for theFFT data being shifted in time by ensuring that the same frequency binsof FFT data from different remote antenna units 112 a-c are addedtogether in bin summers 322 a, 322 b. Although FIG. 3 depicts two binaligners 318 a, 318 b for providing uplink signals to the bin summer 322a and two bin aligners 318 c, 318 d for providing uplink signals to thebin summer 322 b, any number of bin aligners can be used to align binsthat are to be summed at a bin summer. Each of the bin aligners 318 a-dcan provide an uplink signal to a respective one of the gain adjustmentmodules 320 a-d. Each of the gain adjustment modules 320 a-d can adjustthe gain of the uplink signal outputted from a respective one of the binaligners 318 a-d.

Each of the bin summers 322 a, 322 b can sum or otherwise combinefrequency bins from uplink signals in the frequency domain (e.g., OFDMuplink signals). Each of the bin summers 322 a, 322 b can add the samefrequency bins of FFTs from multiple remote antenna units. For example,three FFTs may be obtained from three remote antenna units, where eachFFT includes 1,024 bins. A bin summer can add the first bin from thethree FFTs and can save the sum in the first bin of the FFT sum. The binsummer can add the second bin from the three FFTs and can save the sumin the second bin of the FFT sum. The bin summer can repeat the processuntil all bin values for each of the 1,024 bins of the FFT are added.

The inverse FFT modules 326 a, 326 b can transform FFTs of uplinksignals into the time domain for transmission to the base stations 101a, 101 b. Although FIG. 3 depicts inverse FFT modules 326 a, 326 b fortransforming FFTs into the time domain, the unit 102 can include anysuitable inverse frequency transform device for transforming frequencydomain representations of signals into the time domain. Thedigital-to-analog converters 328 a, 328 b can convert digital uplinksignals in the time domain to analog uplink signals for transmission tobase stations 101 a, 101 b via the RF circuitry 330 a, 330 b included inthe interface section 106.

FIG. 4 is a block diagram depicting an alternative example of a unit102′ that can sub-divide downlink signals into sub-bands in thefrequency domain. The unit 102′ can include downlink filters 402 a, 402b and decimators 404 a, 404 b in the downlink paths. Theanalog-to-digital converters 304 a, 304 b may sample the downlinksignals received from the base stations 101 a, 101 b at a higher ratethan is desirable for representing the signals that occupy the bandwidthof the input RF signal. The decimators 404 a, 404 b can be used toreduce the sampling rate of the digital downlink signal to a rate at ornear the minimum used to represent the downlink signals (e.g., theNyquist rate). Reducing the sampling rate of the digital downlink signalcan conserve transmission resources, such as the available bandwidth ofcommunication links between the unit 102 and the remote antenna units112 a-c. A frequency domain decimation function can be provided toreduce the sampling rate.

Each of the downlink filters 402 a, 402 b can be a low-pass filter thatcan filter the transformed downlink signal outputted from a respectiveone of the FFT modules 306 a, 306 b. Filtering the transformed downlinksignals can prevent or reduce aliasing caused by the decimators 404 a,404 b. The downlink filters 402 a, 402 b can also attenuate signals infrequency bins from the FFT that are not of interest. Attenuating thesignals that are not of interest using the downlink filters 402 a, 402 bcan allow the decimators 404 a, 404 b to reduce aliasing to anacceptable level by removing the attenuated frequency bins from the FFTrepresentations of the downlink signals. Each of the decimators 404 a,404 b can reduce the sampling rate of downlink signals by removing thefrequency bins that have been attenuated.

The unit 102 depicted in FIG. 4 can also include interpolators 406 a,406 b and uplink filters 408 a, 408 b in the uplink paths. Each of theinterpolators 406 a, 406 b can increase the sampling rate of arespective uplink signal outputted from a respective one of the gainadjustment modules 324 a, 324 b. Each of the uplink filters 408 a, 408 bcan filter the signal outputted from a respective one of theinterpolators 406 a, 406 b to prevent or reduce aliasing caused by theinterpolators 406 a, 406 b.

In additional or alternative aspects, frequency transforms of uplinksignals can be used to reduce the bandwidth requirements forcommunicating the uplink signals via the DAS 100. For example, the unit102 can combine uplink signals received by multiple remote antenna units112 a-c into a combined uplink signal for transmission to one or more ofthe base stations 101 a, 101 b. The unit 102 can add or otherwisecombine individual sub-bands of uplink signals from different remoteantenna units 112 a-c. The unit 102 can exclude other sub-bands of theuplink signals from the combined uplink signal. For example, a givensub-band or group of sub-bands from one remote antenna unit may beunexcited. An unexcited sub-band can include a sub-band in which onlynoise without any signal component is received at a remote antenna unit.Unexcited sub-bands may be excluded from a combination of sub-bands inthe same frequency range received from other remote antenna units (e.g.,a squelch operation). In some aspects, sub-bands of interest can beidentified and extracted by a processor 104 of a unit 102 receivinguplink signals from remote antenna units. In other aspects, sub-bands ofinterest can be identified and extracted by a processor of a remoteantenna unit.

FIG. 5 is a block diagram depicting an example of a remote antenna unit112 that can sub-divide uplink signals into sub-bands in the frequencydomain. The remote antenna unit 112 can include an interface section 502and a signal processing section 503. The interface section 502 caninclude a PHY device 504, a deframer 506, and a framer 522. The signalprocessing section 503 can include an interpolator 508, a low-passfilter 510, an inverse FFT module 512, a digital-to-analog converter514, a processor 515, an analog-to-digital converter 516, an FFT module518, a filter 520, and a decimator 524.

In a downlink path, the PHY device 504 can receive packetized downlinkdata streams from the unit 102. The deframer 506 can extract downlinkdata from the packetized downlink data streams. The interpolator 508 canincrease the sampling rate of a digital downlink signal received fromthe deframer 506. The filter 510 can be a low-pass filter or otherfilter that is suitable for preventing or reducing aliasing caused bythe interpolator 508. The inverse FFT module 512 or other suitableinverse frequency transform device can transform digital downlinksignals from the frequency domain into the time domain. Thedigital-to-analog converter 514 can convert the digital downlink signalsto analog downlink signals for transmission to mobile communicationdevices or other terminal devices via suitable RF circuitry.

In an uplink path, the analog-to-digital converter 516 can convertanalog uplink signals to digital uplink signals. The analog uplinksignals can be received using suitable RF circuitry of the remoteantenna unit 112. An FFT module 518 or other frequency transform devicecan transform the digital uplink signals into the frequency domain. Theprocessor 515 can extract or otherwise select sub-bands of interest fromthe uplink signal in the frequency domain. The processor 515 can includeany device suitable for executing program instructions stored in anon-transitory computer-readable medium or other memory device tocontrol operation of the remote antenna unit 112. Examples of processor515 include a microprocessor, an ASIC, an FPGA, or other suitableprocessor.

The filter 520 can be a low-pass filter or other filter that is suitablefor preventing or reducing aliasing caused by the decimator 524. Thedecimator 524 can decrease the sampling rate of the digital uplinksignal for transmission via the DAS 100. The framer 522 can packetizeuplink data received from mobile communication devices for transmissionto the unit 102. The PHY device 504 can transmit packetized uplink datastreams to the unit 102.

In some aspects, one or more of the elements in the signal processingsection 503 can be implemented as software modules executed by theprocessor 104. In additional or alternative aspects, one or more of theelements in the signal processing section 503 can be implemented usingdedicated signal processing circuitry, such as an FPGA. Although FIG. 5depicts the downlink path as including an interpolator 508 and alow-pass filter 510, other implementations are possible. In someaspects, the interpolator 508 and the low-pass filter 510 can beomitted. Although FIG. 5 depicts the uplink path as including the 520and the decimator 524, other implementations are possible. In someaspects, the filter 520 and the decimator 524 can be omitted.

In some aspects, a DAS 100 can be used to communicate orthogonalfrequency-division multiplexing (“OFDM”) signals. Some telecommunicationtechnologies, such as OFDM-based signals transmitted via long-termevolution (“LTE”) systems, use FFTs and inverse FFTs for processingsignals that are communicated with mobile communication devices or otherterminal devices. These types of signals can be sub-divided intosub-bands using an FFT. Each FFT can be aligned with a complete OFDMsymbol.

For example, LTE signals can include multiple resource blocks. Eachresource block may have a size of one FFT bin. Each FFT bin can beencoded via quadrature amplitude modulation (“QAM”) to conveyinformation. A sub-banding unit of a DAS 100 can decompose an LTE signalinto FFT bins. Unused resource blocks may not be transmitted from themaster unit to the remote antenna unit. A guard time (called the cyclicprefix) used between LTE symbols may not convey information. The guardtime or cyclic prefix may not be transmitted from the master unit to theremote antenna unit. Excluding un-used resource blocks or guard timesfrom signals communicated via a DAS 100 can reduce the bandwidth used tocommunicate the signals.

OFDM signals can be communicated in an FFT format. If a known OFDM-basedsignal uses an FFT format, the unit 102 can synchronize the start andstop of FFT frames to correspond with the start and stop of theOFDM-based symbols to be transported. Synchronizing the start and stopof FFT frames to correspond with the start and stop of the OFDM-basedsymbols can include analyzing the OFDM signal with a measurement moduleimplemented in the processor 104 of the unit 102. The measurement modulecan determine the timing of the received signal. The timing of thesignal can be used to control when to start and stop FFT frames.

In additional or alternative aspects, the unit 102 of the DAS 100 canequalize a frequency response throughout the DAS 100. Some analogcircuitry for devices in a DAS 100 can have a frequency response that isnot as constant (or “flat”) as desirable. A desirable frequency responsecan have a magnitude and group delay that is constant across thefrequency spectrum used by the DAS 100. Imperfections in analogcircuitry can introduce ripples and other imperfections across thefrequency spectrum. The unit 102 can execute one or more operations inthe frequency domain to apply the inverse of the frequency response ofthe analog circuitry. Applying the inverse of the frequency response caninclude applying different complex weightings to different binscorresponding to frequencies of interest to compensate for variance ofmagnitude and/or group delay across the frequency spectrum used by theDAS 100. Applying the inverse of the frequency response can equalize thefrequency response throughout the DAS 100.

In some aspects, a magnitude (i.e., weight) associated with each bin ofinterest can be modified by the processor applying a multiplier to thebin. In other aspects, a complex weight can be applied to the bins ofinterest. A complex weight can be used to modify the gain and phase of abin by performing a complex multiplication of the bin and the complexweight.

The foregoing description of aspects and features of the disclosure,including illustrated examples, has been presented only for the purposeof illustration and description and is not intended to be exhaustive orto limit the disclosure to the precise forms disclosed. Numerousmodifications, adaptations, and uses thereof will be apparent to thoseskilled in the art without departing from the scope of this disclosure.Aspects and features from each disclosed example can be combined withany other example. The illustrative examples described above are givento introduce the reader to the general subject matter discussed here andare not intended to limit the scope of the disclosed concepts.

What is claimed is:
 1. A method comprising: sub-dividing a downlinksignal into a sub-divided downlink signal having a plurality of downlinksignal components in a time domain, wherein each of the plurality ofdownlink signal components corresponds to a respective sub-band;determining that at least one component of the plurality of downlinksignal components corresponds to at least one sub-band having data to betransmitted via a distributed antenna system; generating, subsequent todetermining that the at least one component from the sub-divideddownlink signal corresponds to the at least one sub-band, a transformeddownlink signal by performing a frequency transform on the downlinksignal, wherein the transformed downlink signal represents the downlinksignal in a frequency domain; determining that the at least one sub-bandof the transformed downlink signal includes the data to be transmitted;and extracting the at least one sub-band from the transformed downlinksignal for transmission via the distributed antenna system; whereinextracting the at least one sub-band from the transformed downlinksignal comprises: identifying, by a unit in the distributed antennasystem, at least one bin of the transformed downlink signal in thefrequency domain that corresponds to the at least one sub-band,providing, by the unit, data for the at least one bin to at least oneremote antenna unit of the distributed antenna system, identifying, bythe unit, at least one additional bin of the transformed downlink signalin the frequency domain that corresponds to at least one additionalsub-band that lacks data to be transmitted, and discarding additionaldata for the at least one additional bin.
 2. The method of claim 1,wherein performing the frequency transform comprises executing at leastone of a fast Fourier transform algorithm, a discrete Fourier transformalgorithm, and a discrete cosine transform algorithm using the downlinksignal as an input.
 3. The method of claim 1, wherein determining thatthe at least one sub-band of the transformed downlink signal includesthe data to be transmitted comprises: identifying, for each of aplurality of bins of the transformed downlink signal in the frequencydomain, a respective signal level for the bin; and determining that atleast one of the plurality of bins that corresponds to the at least onesub-band has a signal level exceeding a threshold signal level.
 4. Themethod of claim 1, wherein determining that the at least one sub-band ofthe transformed downlink signal includes the data to be transmittedcomprises: receiving a control signal from a base station; identifying,from the control signal, at least one channel used by the base stationto transmit downlink signals; and identifying the at least one sub-bandfrom the at least one identified channel.
 5. The method of claim 1,wherein discarding the additional data for the at least one additionalbin comprises configuring a combiner module of the unit to exclude theadditional data from a combining operation used to route the data forthe at least one bin to the at least one remote antenna unit.
 6. Themethod of claim 1, further comprising: determining that at least oneadditional sub-band lacks data to be transmitted via the distributedantenna system; modifying the transformed downlink signal to exclude theat least one additional sub-band; and reducing a sampling rate of thetransformed downlink signal based on the transformed downlink signalbeing modified to exclude the at least one additional sub-band.
 7. Themethod of claim 1, further comprising: receiving, by the unit of thedistributed antenna system, a plurality of transformed uplink signalsfrom a plurality of remote antenna units of the distributed antennasystem, wherein each of the plurality of transformed uplink signals isgenerated by performing an additional frequency transform on arespective uplink signal to represent the respective uplink signal inthe frequency domain; identifying, by the unit, a subset of sub-bandsfrom the plurality of transformed uplink signals that include uplinkdata to be transmitted via the distributed antenna system; andcombining, by the unit, the subset of sub-bands for transmission to abase station, wherein a combining operation performed by the unitexcludes sub-bands other than the subset of sub-bands that include theuplink data.
 8. A unit comprising: a first interface section configuredto receive a downlink signal from a base station; a signal processingsection configured to sub-divide the downlink signal into a sub-divideddownlink signal having a plurality of downlink signal components in atime domain, wherein each of the plurality of downlink signal componentscorresponds to a respective sub-band; a processor communicativelycoupled to the first interface section and configured: determine that atleast one component of the plurality of downlink signal componentscorresponds to at least one sub-band having data to be transmitted via adistributed antenna system; generate, subsequent to determining that theat least one component from the sub-divided downlink signal correspondsto the at least one sub-band, a transformed downlink signal byperforming a frequency transform on the downlink signal, wherein thetransformed downlink signal represents the downlink signal in afrequency domain, determine that the at least one sub-band of thetransformed downlink signal includes the data to be transmitted, andextract the at least one sub-band from the transformed downlink signal;and a second interface section configured to provide the at least onesub-band extracted from the transformed downlink signal to at least oneremote antenna unit of the distributed antenna system remote from theunit; wherein the processor is configured to extract the at least onesub-band from the transformed downlink signal by performing operationscomprising: identifying at least one bin of the transformed downlinksignal in the frequency domain that corresponds to the at least onesub-band, providing data for the at least one bin to the at least oneremote antenna unit of the distributed antenna system, identifying atleast one additional bin of the transformed downlink signal in thefrequency domain that corresponds to at least one additional sub-bandthat lacks data to be transmitted, and discarding additional data forthe at least one additional bin.
 9. The unit of claim 8, wherein theprocessor is configured to provide the frequency transform by executingat least one of a fast Fourier transform algorithm, a discrete Fouriertransform algorithm, and a discrete cosine transform algorithm using thedownlink signal as an input.
 10. The unit of claim 8, wherein theprocessor is configured to determine that the at least one sub-band ofthe transformed downlink signal includes the data to be transmitted byperforming operations comprising: identifying, for each of a pluralityof bins of the transformed downlink signal in the frequency domain, arespective signal level for the bin; and determining that at least oneof the plurality of bins that corresponds to the at least one sub-bandhas a signal level exceeding a threshold signal level.
 11. The unit ofclaim 8, wherein the processor is configured to discard the additionaldata for the at least one additional bin by providing a control signalto a combiner module of the unit, wherein the control signal isconfigured to configure the combiner module to exclude the additionaldata from a combining operation used to route the data for the at leastone bin to the at least one remote antenna unit.
 12. The unit of claim8, wherein the processor is further configured to determine that atleast one additional sub-band lacks data to be transmitted via thedistributed antenna system, wherein the signal processing section isfurther configured to: modify the transformed downlink signal to excludethe at least one additional sub-band; and reduce a sampling rate of thetransformed downlink signal based on the transformed downlink signalbeing modified to exclude the at least one additional sub-band.
 13. Theunit of claim 8, wherein the second interface section is furtherconfigured to receive a plurality of frequency-transformed uplinksignals from a plurality of remote antenna units of the distributedantenna system; wherein the processor is further configured to identifya subset of sub-bands from the plurality of transformed uplink signalsthat include uplink data to be transmitted via the distributed antennasystem; wherein the signal processing section is further configured tocombine the subset of sub-bands for transmission to the base station andexcluding sub-bands other than the subset of sub-bands that include theuplink data.
 14. A distributed antenna system comprising: a unitconfigured to: receive a downlink signal, sub-divide the downlink signalinto a sub-divided downlink signal having a plurality of downlink signalcomponents in a time domain, wherein each of the plurality of downlinksignal components corresponds to a respective sub-band, determine thatat least one component of the plurality of downlink signal componentscorresponds to at least one sub-band having data to be transmitted viathe distributed antenna system, generate, subsequent to determining thatthe at least one component from the sub-divided downlink signalcorresponds to the at least one sub-band, a transformed downlink signalby performing a frequency transform on the downlink signal, wherein thetransformed downlink signal represents the downlink signal in afrequency domain, determine that the at least one sub-band of thetransformed downlink signal includes the data to be transmitted, extractthe at least one sub-band from the transformed downlink signal, andprovide the at least one sub-band extracted from the transformeddownlink signal to at least one remote antenna unit; and the at leastone remote antenna unit configured to: receive, from the unit, the atleast one sub-band extracted from the transformed downlink signal,generate a wireless RF signal based on the at least one sub-band, andtransmit the wireless RF signal to a terminal device; wherein the unitis configured to extract the at least one sub-band from the transformeddownlink signal by performing operations comprising: identifying atleast one bin of the transformed downlink signal in the frequency domainthat corresponds to the at least one sub-band, providing data for the atleast one bin to the at least one remote antenna unit, identifying atleast one additional bin of the transformed downlink signal in thefrequency domain that corresponds to at least one additional sub-bandthat lacks data to be transmitted, and discarding additional data forthe at least one additional bin.
 15. The distributed antenna system ofclaim 14, wherein the unit is configured to perform the frequencytransform by executing at least one of a fast Fourier transformalgorithm, a discrete Fourier transform algorithm, and a discrete cosinetransform algorithm using the downlink signal as an input.
 16. Thedistributed antenna system of claim 14, wherein the unit is configuredto determine that the at least one sub-band of the transformed downlinksignal includes the data to be transmitted by performing operationscomprising: identifying, for each of a plurality of bins of thetransformed downlink signal in the frequency domain, a respective signallevel for the bin; and determining that at least one of the plurality ofbins that corresponds to the at least one sub-band has a signal levelexceeding a threshold signal level.